There has recently been increasing use of UV-curing organic glass printing inks, which have to be post-treated with heat after curing, instead of the ceramic (inorganic) inks previously used, for the printing of glass. This opens up the possibility of managing without heavy metals, having access to a wider range of colours (standard and mixed shades) and achieving greater brilliance and a higher degree of gloss. Another advantage is the possibility of evaluating the shade immediately after UV curing. In contrast, with inorganic inks evaluation is only possible after stoving. Furthermore, they offer process advantages such as a more rapid tempering process (e.g. approx. 20 min compared with 90 min tempering) and a reduced tempering temperature (e.g. 160° C. compared with 500-700° C.), which reduces the process costs compared with processes using inorganic glass printing inks. However, the organic printing inks themselves are more expensive than conventional inorganic inks for printing glass.
The milder process conditions in the printing process using organic glass printing inks ensure that the internal pressure resistance of bottles is not impaired. For the printing of glass that has to be post-treated at high temperatures, such as formed glass for cars or glass with high-gloss gold printing, however, the organic glass printing inks are unsuitable.
With the organic printing inks for glass currently available, the mechanical resistance, opacity and water resistance can present problems, with possible difficulties particularly in obtaining adequate scratch resistance.
The organic printing inks for glass are printed after a pre-treatment of the glass, e.g. by flaming, in a two-component printing process, i.e. with an adhesion promoter that has to be mixed into the printing ink, and are then post-tempered at temperatures of e.g. 140°-200° C. over a specific period.
It would be desirable to simplify this printing process, thus saving costs. One way of simplifying it would be to remove the need to print with two components, i.e. to use an adhesion promoter. Another would consist in rendering a post-tempering superfluous.
WO 99/06336 discloses a process for printing glass in which a glass ink comprising a bisphenol A epoxy resin is printed on to a glass substrate and then cured by radiation, with the optional use of an adhesion promoter.
However, the inventor has found that a glass printing ink of this type and a process of this type does not, without additional measures, provide adequate scratch resistance and also fails to provide adequate adhesion of the glass ink to the glass substrate and adequate water resistance. Accordingly, in all the examples in WO 99/06336, both an adhesion promoter is used and post-tempering is performed at a temperature of 150 to 200° C. U.S. Pat. No. 5,656,336 A discloses a similar process, which also suffers from the same deficiencies.
The present invention is directed to resolving these and other matters.